Recently, there have been increased opportunities of using electrophotographic copiers or printers in the field of printing or color printing. There is a strong trend of requiring high quality digital black-and-white or color images in such the field of printing or color printing. In response to such a requirement was proposed formation of high precision digital images by use of a short wavelength laser light. However, the current condition is that even when forming a precise electrostatic latent image on an electrophotographic photoreceptor by use of a short wavelength laser light and reducing the exposure diameter, the finally obtained electrophotographic image cannot achieve sufficiently high image quality.
The cause thereof is due to the fact that a photosensitive characteristic of an electrophotographic photoreceptor or an electrostatic characteristic of a developer toner is not fully provided with characteristics necessary for precise latent dot image formation or toner image formation.
In other words, organic photoreceptors (hereinafter, also denoted simply as a photoreceptor), which were developed as an electrophotographic photoreceptor used for conventional long wavelength lasers, were inferior in sensitivity characteristics and produced problems such that imagewise exposure with a short wavelength laser light at a reduced dot diameter resulted in an unclear dot latent image, rendering it difficult to obtain a satisfactory dot image.
There have been known anthanthrone pigments and pyranthrone compounds as a charge generation material of a photoreceptor for a short wavelength laser, as described in, for example, JP-A No. 2000-47408 (hereinafter, the term JP-A refers to Japanese Patent Application Publication).
Polycyclic quinone pigments such as anthanthrone pigments, as described in the foregoing patent document have no description of having been subjected to a special treatment and it is assumed to use commercial available ones. However, characteristics such as sensitivity, achieved by use of such commercially available pigments were difficult in satisfying sufficient sensitivity or a high-speed characteristic for high-speed printers or copiers using a short wavelength laser.
To enhance sensitivity, as is well known, a charge generation material is granulated to form a charge generation layer having an enhanced density of the charge generation material. However, application of this granulation technique to a photoreceptor used for a short wavelength laser achieves improved sensitivity itself but tends to produce image defects due to memory generated by repetition of electrostatic-charging in the step of charging or transfer during image formation or due to minute electric charge leakage.